In a motor vehicle equipped with an automated gearshift transmission, such as an automated shift transmission, an automated dual-clutch transmission or a converter automatic transmission, during normal operation, i.e. during driving operation governed solely by control actions of the driver, in particular deflection of the accelerator pedal, shift operations are controlled as a function of operating parameters detected by sensors or calculated from sensor data, such as the actual travel speed, the actual driving resistance, the actual engine torque (MM—Ist) and the actual running speed of the drive engine and the power demanded by the driver as manifested by the actual current deflection of the accelerator pedal, with reference to shift characteristics stored in a data memory of the associated transmission control unit.
To make things easier for the driver, however, for a long time driver-assistance systems called Tempomat or CC systems (CC=Cruise Control) have been known, which enable driving speeds to be set by the driver, which are maintained automatically using an operating unit. In recent years such driver-assistance systems have been extended further by a distance regulation function and thereby developed further into Tempomat distance-regulating or ACC systems (ACC=Adaptive Cruise Control).
When the ACC system is switched on, a speed regulation mode is activated at first, so long as an area of the road ahead of one's own motor vehicle is free from other vehicles. In this speed regulation mode the drive engine and existing permanent braking devices, such as an engine brake and a permanent brake made for example as a retarder, are controlled in such manner that a desired speed, specified by the driver, is largely maintained. By means of a sensor device, preferably comprising at least one radar sensor, the area ahead of one's own motor vehicle is constantly scanned, vehicles driving ahead are recognized, and their driving direction, distance away, and relative speed are determined.
If a vehicle driving ahead in the same lane more slowly is getting too close, a distance regulation mode of the ACC system is activated, in which a required deceleration is calculated, which is necessary in order to follow the vehicle ahead at a distance that can previously be input to the operating unit by the driver. To produce this required deceleration the drive engine, the permanent brake devices and if necessary also the service brakes of the motor vehicle are appropriately controlled so that the specified distance from the vehicle driving ahead will largely be maintained. Thereafter, by virtue of appropriately controlling the drive engine, the permanent brakes and the service brakes, one's own motor vehicle follows the vehicle driving ahead of it at the specified distance. When the vehicle ahead is no longer there, i.e. it has turned off the road or accelerated to a speed higher than the specified speed, the ACC system reverts to the speed regulation mode (Tempomat operation). Other than by manually switching off the ACC system, it can also be deactivated at least temporarily by control actions by the driver, such as overriding the current power setting of the drive engine by actuating the accelerator pedal, a definite actuation of the brake pedal, and a steering movement that results in leaving the lane.
In the speed regulation mode and in the distance regulation mode, the control of a gearshift transmission, i.e. the determination of the target gear or gear change interval in each case and the associated shift or target engine speed for initiating upshifts and downshifts, takes place only indirectly as a function of the speed or distance regulation of the ACC system, namely as a function of the actual torque demand (MM—Soll) on the drive engine and other operating parameters influenced by control or regulating actions of the ACC system.
In this, it has already been shown in CC systems comprising only a speed regulation mode that in certain operating situations the use of the respective torque demand (MM—Soll) on the drive engine for controlling the gearshift transmission is inappropriate, because thereby, if there is a large speed difference (Δv=vSoll−vIst) between the nominal speed (vSoll) and the current actual speed (vIst) unnecessary fluctuating shifts, i.e. repeated upshifts and downshifts can be triggered. Disadvantageously, these unnecessary shifts result in higher fuel consumption, greater wear of the shift elements, and deterioration of the driving comfort of the motor vehicle concerned.
To avoid unnecessary traction downshifts a method is proposed, for example in DE 101 15 052 A1, according to which, in traction operation, the torque demand (MM—Soll>0) of an ACC system for controlling the drive engine when a speed or distance regulation mode is activated for the control of the associated gearshift transmission in general, is restricted for example to a value of 80% (MM—mod=0.8*MM—Soll), and this restriction is lifted only under certain operating conditions. The restriction is lifted when the difference (ΔM=MM—Soll−MM—Ist) between the torque demand (MM—Soll) and the actual torque (MM—Ist) of the drive engine at the time exceeds a first limit value, and/or when the actual torque (MM—Ist) exceeds a second limit value close to the maximum torque (MM—Max) of the drive engine. With this known method a possibly necessary rapid acceleration of the motor vehicle should still be possible.
When the distance regulation mode is active another problem can arise if, to avoid a dangerous approach to a vehicle driving ahead more slowly, the service brake is actuated and when this braking action makes the transmission control inaccessible. In such a case the driving resistance is mistakenly determined as too large by the amount of the braking torque of the service brake, which can result is unnecessary thrust downshifts at shift speeds that are too high.